There are a lot of good documents and white papers on why use the QVD files and how. In regards to your question, I'd do a two step load, like the following:

// This loads the existing data in your QVD file
Original:
LOAD *
FROM File.qvd (qvd);
// This loads today data and appends its records
// to the table above
FromSource:
CONCATENATE (Original) LOAD *;
SQL SELECT *
FROM Database
WHERE Date = '20111220'; // Or whatever source or condition you use to get new records
// Now both the old data and today data are stored into the same QVD, so tomorrow
// the LOAD will be the same
STORE Original INTO File.qvd (qvd);